OP, you also might find that focusing on some of the non-verbal language helps too (and makes the verbal language sound more firm--both in how you speak and in how it's perceived).
I work with (supervise) young people who are often at their first job, they're in a quasi-authoritative position, and the position attracts young people who are going into elem. education (who, if I'm going to egregiously make a gross stereotype, tend to be a particularly soft-spoken lot)--and when needed, we have mini-sessions of "Dawbs's lessons on commanding authority/assertiveness/dealing w/ creepy people (or people who claim they don't take direction from women well)"
What your parents tried to teach you about good posture is a good place to start--shoulders square, straight spine, don't slouch.
If you're standing while speaking, take a slightly wider stance. You know the stance people tell you to take if someone keeps pushing their shopping cart into your heel? when you take a 1/2 step forward but still have your leg behind you? do that. (this is opposite of what young ladies were taught about good posture--make yourself take up more space!)
If someone is questioning your authority/being creepy, I find that 'look big' translates well from the animal kingdom--have you arms at your sides, but a little bit farther way than normal, elbows slightly bent, etc. Not necessarily hands on hips, but hanging loose and wide at the side
(Look, science: http://www.gsb.stanford.edu/news/bmag/sbsm0711/kn-dominance.html
on the topic
Don't be apologetic or, like someone else said, do the 'uptalk' that makes your statement into a question. You can say please and have it not be a question. "Don, can you please take out the trash?" can be replaced with "Don, take out the trash please" Almost the same, minus the question mark--tone of voice makes a difference in that. And giving the expected deadline as a statement ("I expect X to be done by 3) followed by "is that possible?" or "do you forsee any problems with that?" instead of "can you do this by 3?" is a world of difference
Good eye contact is helpful.
I find looking and posing assertively makes me default to more assertive language --and even if my language doesn't change, the perception does.
(Fake it 'til you make it!)